Ideal (Subtractive) Pressuretrol Settings?

SwedishPimple

What are the ideal settings for the Honeywell subtractive pressuretrol found on a gas fired Burnham IN steam boiler?



We recently had a new boiler installed in our 1929 bungalow. This is a one pipe steam system with a single continuous loop that serves a total of seven radiators distributed on 3 upper floors (the house is small, but tall).



Anyway... we would like to set the pressuretrol to a level that allows the system to run as efficiently as possible, while still delivering heat throughout the house.



The current settings on the pressuretrol are:

Main - ~1.8 psi

Diff - ~1.5 psi



I this correct? and/or Is there an ideal range for these settings?



Also, can someone explain the whole diff is subrtractive thing to me?



Thanks much!



S

Jamie Hall

You probably

can't do much better than what you have with a pressuretrol.  The cutout pressure -- that 1.8 psi -- is a bit higher than I would like to see (1.5 psi would be better) but pressuretrols can get a little flaky down in that pressure range, so that is probably as good as it's going to get.  Myself I might set the differential a little smaller -- 1 psi rather than 1.3 -- again simply because pressuretrols are a little flaky at very low pressures.



The way these things work is that the main setting determines the pressure at which the pressuretrol turns off the power to the burner.  When the pressure rises, eventually the sensing switch will turn off.  Then, as the pressure falls, eventually the pressure will drop to the point where the switch turns back on.  The differential setting determines the difference between that off pressure and the on pressure.  It is called subtractive, because the difference is applied between the off pressure and the on pressure -- so the on pressure is found by subtracting the differential from the main off pressure.

SwedishPimple

Thanks

Thanks for your reply.... I think I finally understand this now. I'll try running at the following settings: 1.5 PSI for the main and 1.0 PSI for the differential.